Strain balanced laser diode

Abstract

According to the concepts of the present disclosure, laser diode waveguide configurations are contemplated where the use of Al in the waveguide layers of the laser is presented in the form of InGaN/Al(In)GaN waveguiding superstructure comprising optical confining wells (InGaN) and strain compensating barriers (Al(In)GaN). The composition of the optical confining wells is chosen such that they provide strong optical confinement, even in the presence of the Al(In)GaN strain compensating barriers, but do not absorb lasing emission. The composition of the strain compensating barriers is chosen such that the Al(In)GaN exhibits tensile strain that compensates for the compressive strain of InGaN optical confinement wells but does not hinder the optical confinement.

1. A laser diode comprising a semi-polar GaN substrate, an active region, a waveguiding region, and upper and lower cladding regions, wherein:
 the semi-polar GaN substrate is cut along a semi-polar crystal growth plane; 
 the active region is configured for electrically-pumped stimulated emission of photons at a lasing wavelength λ C ; 
 the waveguiding region comprises at least one waveguiding superstructure; 
 the waveguiding superstructure comprises a plurality of In y Ga 1-y N optical confining wells of well thickness a and a plurality of intervening Al x In z Ga 1-x-z N strain compensating barriers of barrier layer thickness b defining a strain compensated structure, where x, y and z approximate the relations 0.02≦x≦0.40, 0.05≦y≦0.35, and 0≦z≦0.10; 
 the intervening strain compensating barriers comprise sufficient Al to compensate for strain introduced by the optical confining wells; and 
 the respective active, waveguiding, and upper and lower cladding regions are formed as a multi-layered laser diode over the semi-polar crystal growth plane of the semi-polar GaN substrate such that the waveguiding region guides the stimulated emission of photons from the active region, and the cladding region promotes propagation of the emitted photons in the waveguiding region. 
 
     
     
       2. A laser diode as claimed in  claim 1  wherein:
 the intervening strain compensating barriers comprise sufficient Al to compensate for strain introduced by the optical confining wells to a strain compensation percentage θ, where θ>0 and θ=1 represents complete strain compensation; 
 the respective thicknesses a, b of the optical confining wells and the intervening strain compensating barriers satisfy the relation
   (0.1y)η≈θ(0.039x+0.1z)(1−η)
 
 
 where x, y and z are averaged over the waveguiding superstructure, η is the periodic or aperiodic In y Ga 1-y N confining well duty cycle or average duty cycle in the waveguiding 
 
       
         
           
             
               superstructure 
               , 
               
                 
                   and 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     a 
                     b 
                   
                 
                 = 
                 
                   
                     η 
                     
                       1 
                       - 
                       η 
                     
                   
                   . 
                 
               
             
           
         
       
     
     
       3. A laser diode as claimed in  claim 1  wherein the waveguiding superstructure is characterized by an absorption edge wavelength λ W  that approximates the relation 10 nm≦(λ C −λ W )≦60 nm. 
     
     
       4. A laser diode as claimed in  claim 1  wherein:
 the waveguiding superstructure is configured as a passive MQW waveguide layer; 
 the optical confining wells comprise quantum wells; and 
 the strain compensating barriers comprise quantum well barrier layers. 
 
     
     
       5. A laser diode as claimed in  claim 4  wherein the waveguiding superstructure is characterized by an absorption edge wavelength λ W  that approximates the relation 10 nm≦(λ C −λ W )≦40 nm. 
     
     
       6. A laser diode as claimed in  claim 1  wherein In y Ga 1-y N optical confining wells provide compressive strain and the Al x In z Ga 1-x-z N strain compensating barriers provide tensile strain sufficient to compensate for a majority of the compressive strain provided by the In y Ga 1-y N optical confining wells. 
     
     
       7. A laser diode as claimed in  claim 6  wherein the average refractive index of the waveguiding superstructure is higher than the GaN substrate and the upper and lower cladding layers. 
     
     
       8. A laser diode as claimed in  claim 1  wherein the waveguiding superstructure comprises a plurality of Al x In z Ga 1-x-z N strain compensating barriers which collectively define a thickness less than approximately 300 nm. 
     
     
       9. A laser diode as claimed in  claim 1  wherein the well thickness a is between approximately 2 nm and approximately 5 nm. 
     
     
       10. A laser diode as claimed in  claim 1  wherein the well thickness a does not exceed approximately 60 nm. 
     
     
       11. A laser diode as claimed in  claim 1  wherein x approximates the relation 0.05≦x≦0.20 for at least one strain compensating barrier. 
     
     
       12. A laser diode as claimed in  claim 1  wherein z approximates the relation 0<z≦0.10 for at least one strain compensating barrier. 
     
     
       13. A laser diode as claimed in  claim 1  wherein:
 the intervening strain compensating barriers comprise sufficient Al to compensate for substantially all strain introduced by the optical confining wells, such that the strain compensation percentage θ is above approximately 0.9; and 
 y approximates the relation 0.15≦y≦0.35 for at least one optical confining well. 
 
     
     
       14. A laser diode as claimed in  claim 1  wherein y is in a range that is partially defined by the strain compensation percentage θ and approximates the relation 0.15θ≦y≦0.35 for at least one optical confining well. 
     
     
       15. A laser diode as claimed in  claim 1  wherein the intervening strain compensating barriers comprise sufficient Al to compensate for a majority of strain introduced by the optical confining wells, such that the strain compensation percentage θ is above approximately 0.5. 
     
     
       16. A laser diode as claimed in  claim 1  wherein the waveguiding region of the laser diode comprises at least one waveguiding superstructure on each side of the active region in the multi-layered laser diode such that each waveguiding superstructure guides the stimulated emission of photons from the active region. 
     
     
       17. A laser diode as claimed in  claim 1  wherein the lasing wavelength λ C  is between approximately 500 nm and approximately 540 nm and the absorption edge wavelength λ W  is between approximately 430 nm and approximately 530 nm. 
     
     
       18. A laser diode as claimed in  claim 1  wherein the absorption edge wavelength λ W  approximates the relation 10 nm≦(λ C −λ W )≦20 nm. 
     
     
       19. A laser diode as claimed in  claim 1  wherein:
 the laser diode further comprises one or more additional confinement layers in the form of a bulk InGaN layer or an InGaN/GaN superlattice configured to improve optical confinement at the lasing wavelength λ C ; and 
 the respective thicknesses of the bulk InGaN layer or an InGaN/GaN superlattice are small enough to prevent strain-induced relaxation in the laser diode. 
 
     
     
       20. A laser diode as claimed in  claim 19  wherein the waveguiding superstructure is sandwiched between the active region and the additional confinement layers. 
     
     
       21. A laser diode as claimed in  claim 1  wherein the laser diode comprises an asymmetric waveguide core comprising the active region and the waveguiding superstructure configured to shift an optical mode of the laser to an n-side of the laser diode. 
     
     
       22. A laser diode comprising a semi-polar GaN substrate, an active region, a waveguiding region, and upper and lower cladding regions, wherein:
 the semi-polar GaN substrate is cut along a semi-polar crystal growth plane; 
 the active region is configured for electrically-pumped stimulated emission of photons at a lasing wavelength λ C ; 
 the waveguiding region comprises at least one waveguiding superstructure; 
 the waveguiding superstructure comprises one or more In y Ga 1-y N optical confining wells of well thickness a and one or more intervening Al x In z Ga 1-x-z N strain compensating barriers of barrier layer thickness b defining a strain compensated structure, where x, y and z approximate the relations 0.02≦x≦0.40, 0.05≦y≦0.35, and 0≦z≦0.10; 
 the intervening strain compensating barriers comprise sufficient Al to compensate for a majority of strain introduced by the optical confining wells to a strain compensation percentage θ, where θ>0 and θ=1 represents complete strain compensation; 
 the respective thicknesses a, b of the optical confining wells and the intervening strain compensating barriers satisfy the relation
   (0.1y)η≈θ(0.039x+0.1z)(1−η)
 
 
 where x, y and z are averaged over the waveguiding superstructure, η is the periodic or aperiodic In y Ga 1-y N confining well duty cycle or average duty cycle in the waveguiding superstructure, and 
 
       
         
           
             
               
                 
                   a 
                   b 
                 
                 = 
                 
                   η 
                   
                     1 
                     - 
                     η 
                   
                 
               
               ; 
             
           
         
       
       and
 the respective active, waveguiding, and upper and lower cladding regions are formed as a multi-layered laser diode over the semi-polar crystal growth plane of the semi-polar GaN substrate such that the waveguiding region guides the stimulated emission of photons from the active region, and the cladding region promotes propagation of the emitted photons in the waveguiding region. 
 
     
     
       23. A laser diode comprising a semi-polar GaN substrate, an active region, a waveguiding region, and upper and lower cladding regions, wherein:
 the semi-polar GaN substrate is cut along a semi-polar crystal growth plane; 
 the active region is configured for electrically-pumped stimulated emission of photons at a lasing wavelength λ C ; 
 the waveguiding region comprises at least one waveguiding superstructure and is characterized by an absorption edge wavelength λ W  that approximates the relation 10 nm≦(λ C −λ W )≦20 nm; 
 the waveguiding superstructure is configured as a passive MQW waveguide layer and comprises a plurality of In y Ga 1-y N optical confining wells of well thickness a and intervening Al x In z Ga 1-x-z N strain compensating barriers of barrier layer thickness b defining a strain compensated structure, where x, y and z approximate the relations 0.02≦x≦0.40, 0.15≦y≦0.35, and 0<z≦0.10; 
 the optical confining wells comprise quantum wells; 
 the strain compensating barriers comprise quantum well barrier layers; 
 the intervening strain compensating barriers comprise sufficient Al to compensate for a majority of strain introduced by the optical confining wells to a strain compensation percentage θ, where θ>0 and θ=1 represents complete strain compensation; 
 the respective thicknesses a, b of the optical confining wells and the intervening strain compensating barriers satisfy the relation
   (0.1y)η≈θ(0.039x+0.1z)(1−η)
 
 
 
       where x, y and z are averaged over the waveguiding superstructure, η is the In y Ga 1-y N confining well duty cycle in the waveguiding superstructure, and 
       
         
           
             
               
                 
                   a 
                   b 
                 
                 = 
                 
                   η 
                   
                     1 
                     - 
                     η 
                   
                 
               
               ; 
             
           
         
         the tensile strained Al x In z Ga 1-y-z N has a lower refractive index than the GaN substrate and the upper and lower cladding layers; 
         the average refractive index of the waveguiding superstructure is higher than the GaN substrate and the upper and lower cladding layers; and 
         the respective active, waveguiding, and upper and lower cladding regions are formed as a multi-layered laser diode over the semi-polar crystal growth plane of the semi-polar GaN substrate such that the waveguiding region guides the stimulated emission of photons from the active region, and the cladding region promotes propagation of the emitted photons in the waveguiding region.